Definition
Treasury bills are sold at public auctions every week. Competitive bids at these auctions determine the interest rate paid on each issue. A group of securities dealers, known as primary dealers, are authorized and obligated to submit competitive tenders at Treasury auctions. Dealers can hold the bills, resell the bills to their clients or trade them with other securities firms. Typically, the New York Fed approves about 20 securities firms to be primary dealers but that number dropped sharply during the 2008 financial crisis as some were merged into other firms or went bankrupt. The Fed has been rebuilding that number regularly and the latest list can be found here. Since these are public auctions, the Treasury must announce the size, date and time of the auction every week.

The Treasury announced at its quarterly refunding announcement on August 1, 2018 that they were adding eight week bills and said they would begin to be auctioned starting on October 16. Eight-week bills will be announced on Monday for a Tuesday auction and are issued (settled) on Tuesday of the same week. If a Monday is a banking holiday, the bills are auctioned on Wednesday. Unlike other bills which settle on Thursday, the new 2-month bill will settle on Tuesday, with the first auction to be announced on October 15. (Department of the Treasury)
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